[en] In this paper, we present how the Friedrichs–Lee model could be extended to the relativistic scenario and be combined with the relativistic quark pair creation model in a consistent way. This scheme could be applied to study the "unquenched" effect of the meson spectra. As an example, if the lowest J${}^{PC}$=0${}^{++}$ ($u\stackrel{¯<!--\; ??\; -->}{u}$+$d\stackrel{¯<!--\; ??\; -->}{d}$)/$\sqrt{2}$ bound state in the potential model is coupled to the ππ continuum, two resonance poles could be found from the scattering amplitude for the continuum states. One of them could correspond to the f${}_0$(500)/σ and the other probably f${}_0$(1370). This scheme might shed more light on why extra states could appear in the hadron spectrum other than the prediction of the quark potential model.